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the pin border), see Figure 6 and these fi brils
can extend over several ripples. The fi bril
formation is more marked for the unirradiated
than for the irradiated material and then for
the crosslinked materials.
For the XLPEs the homogeneous ripple-like
microstructure can be seen overall the pin
surface. The fi bril formation is less marked
for the XLPEs than for the unirradiated and
irradiated UHMWPEs. Compared to the noncrosslinked
UHMWPEs, XLPEs show much
less particle formation and smaller size of
the fi brils. As for the unidirectional tests, the
fi brils are much likely rounded and smaller
size, which under the SEM appear as white
particles forming on the ripples of the microstructure.
As explained for the unidirectional
wear tests, this particle formation in XLPEs is
caused by their lower plasticity behaviour.
From the size of the fi brils forming on both
non-crosslinked UHMWPEs and XLPEs, the
size of the particles forming the UHMWPE
debris can be estimated. Figures 5i to 5l
and 6 show that the size of the UHMWPE
particles detaching from the worn surfaces
are ranging from submicron to more than
one micron. It has been found that in Total
Hip Replacements the majority of UHMWPE
particles are less than one micron in length
[13]. Studies of wear particles retrieved from
periprosthetic tissues and analyses of worn
polyethylene surfaces have demonstrated
fi ndings that are consistent with an average
particle size in the 0.5 micrometers diameter
range. [3,7].
Test parameter Value
Contact geometry Cylinder-on-fl at (non-conformal)
Frequency 1 Hz
Relative surface velocity 100 rpm
Contact area Line
Load applied ~150 N (15 Kg)
Contact stresses 5 MPa
Test length 34 Km (123,000 disk rotations)
Lubricant 30 mg/ml initial protein content
Temperature Room
Counterface component CoCrMo alloy
UHMWPE component (GUR 1050) Non-treated
Crosslinked I (γ-sterilised + stabilised I)
Crosslinked II (γ-sterilised + stabilised II)
References
1. Miller D.A., Herrington S.M, Higgins J.C.,
Schroeder D.W, “UHMWPE polyethylene
in total joint replacement: History and
current tecnology” in Encyclopaedic Handbook
of biomaterials and bioengineering,
Part B: Applications, Volume 1, (DL.Wise
et alumina; editors), Marcel Dekker, Inc.,
pp. 665-688, 1995.
2. Greer K.W., Hamilton J.V., Cheal E.J.,
“Polyethylene wear in orthopaedics” in
Encyclopaedic Handbook of biomaterials
and bioengineering, Part B: Applications,
Volume 1, (DL.Wise et alumina; editors),
Marcel Dekker, Inc., pp. 613-638, 1995.
3 Murray D., Rushton N., “Macrophages
stimulate bone resorption when they
phagocytose particles”, J. Bone and Joint
Surgery 72-B, 988-992, 1990.
4. Howie D., McGee M., “Wear and osteolysis
in relation to prostheses design and materials”
in Medical applications of Titanium
and its alloys (ASTM STP 1272), 1996.
5. McGee M., Howie D., Neale S., Haynes
D., Pearcy M., “The role of polyethylene
wear in joint replacement failure”, Proc.
Instn. Mech. Engrs. Part H Vol. 211, 65-
72, 1997.
6. Green T., Fisher J., Stone M., Wroblewski
B., Ingham E., “Polyethylene particles of a
‘critical size’ are necessary for the induction
of cytokines by macrophages in vitro”,
Biomaterials 19, 2297-2302, 1998.